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Instant Expert: The Nuclear Age

By John Pickrell

The Earth exploded into the nuclear age on 16 July 1945.On that day, the US tested a completely new type of weapon in the New Mexico desert. Crafted from a tennis-ball-sized plutonium sphere, the Trinity bomb produced an explosion equivalent to 20,000 tonnes of TNT.

Sixty years on, tens of thousands of tonnes of plutonium and enriched uranium have been produced. The global nuclear arsenal stands at about 27,000 bombs. Nine countries very probably possess nuclear weapons, while 40 others have access to the materials and technology to make them.

But nuclear technology has also been used for peaceful means. The first nuclear reactor to provide electricity to a national grid opened in England in 1956. Now, 442 reactors in 32 nations generate 16% of the world’s electricity.

Nuclear power has been championed as a source of cheap energy. But this was undermined at the end of the 20th century by high-profile reactor accidents, the problems of radioactive waste disposal, competition from more-efficient electricity sources and unavoidable links to nuclear weapons proliferation. Nonetheless, growing evidence for global warming had led some to argue that nuclear power is the only way to generate power without emitting greenhouse gases.

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Splitting atoms

The first steps towards unleashing the power within the atomic nucleus began in 1905 when Albert Einstein established that even tiny quantities of mass are equivalent to immense amounts of energy, through his equation E&equals;mc2. In 1938, Germans Otto Hahn and Fritz Strassman split inherently unstable uranium atoms by bombarding them with neutrons. The following year, Lise Meitner and Otto Frisch elucidated this process of nuclear fission, in which atomic nuclei are split to create nuclei of lighter elements, with neutrons and energy as by-products.

Uranium is the heaviest element found in nature in more than trace amounts, and natural ores contain two isotopes&colon; U-238 and U-235. Only U-235, which makes up just 0.7% of ores, is fissile. So the uranium must be “enriched” to remove U-238 – highly enriched weapons-grade uranium can be up to 90% U-235.

When bombarded with neutrons, U-235 atoms absorb them and become unstable. They split to form two smaller nuclei of other elements and neutrons. Some of the mass is converted to energy in the form of gamma radiation and heat. Because only one neutron is needed to trigger fission and two or three are released, a chain reaction can result. This reaction is uncontrolled in an atomic bomb but tightly controlled in a nuclear reactor.

Dropping the bomb

The Hiroshima bomb was made of enriched uranium, compressed by detonating explosives to achieve a supercritical mass. The Nagasaki bomb was made of plutonium, which is also fissile. Plutonium is produced in the spent fuel of a nuclear reactor, via the irradiation of uranium 238. It can be extracted to create weapons.

The NPT aimed to limit the spread of atomic weapons and bound the five original nuclear weapons states to sharing nuclear technology and materials for peaceful means – mainly through US and Russian disarmament, the treaty has achieved the decommisioning of 38,000 warheads since 1986.

Controlling the remains of the Soviet Union’s vast and poorly protected nuclear arsenal is another great challenge. The G8 have repeatedly pledged billions of dollars to help safeguard the massive stockpile.

Unlike in atomic weapons, nuclear reactors must tightly control the fission chain reaction. To prevent a runaway reaction, control rods are interspersed with the fuel rods of uranium or plutonium. The control rods absorb neutrons, and can be lowered into the reactor core to regulate energy output. A moderating substance, such as water or graphite, surrounds the rods, slowing neutrons emitted by the reaction, and deflecting them back to the centre.

A coolant circulates around the core, and is pumped to a heat exchanger, where water becomes steam and drives electricity-generating turbines. Advanced gas-cooled reactors, such as those used in the UK, use compressed carbon dioxide as the coolant. Light-water, heavy-water and pressurised-water reactors, use water as moderator and coolant.

These reactors are inherently inefficient, only utilising around 1% of the energy stored in the uranium fuel. To overcome this inefficiency and minimise nuclear waste, some countries re-process nuclear fuel. The Sellafield facility in the UK is the largest re-processing facility in the world, but has suffered many problems.

More advanced (but less safe) breeder reactors use liquid sodium metal as a coolant and generate plutonium fuel. Breeder reactors such Superphénix in France, Dounreay in the UK, Monju in Japan and planned reactors in India, can utilise up to 75% of the energy contained in uranium. New miniature Rapid-L reactors might one day even provide power in the basements of apartment blocks and “take-away”, portable reactors are planned for the future.

Going critical

However, several high profile accidents damaged public confidence in nuclear power. The worst US nuclear accident was in 1979, when a cooling system malfunctioned at Three Mile Island in Pennsylvania. The reactor melted down, releasing radioactive gas into the environment. There are now concerns about safety with other ageing US reactors.

The world’s most catastrophic nuclear accident happened in 1986, at Chernobyl in Ukraine. Control rods were withdrawn from the reactor in misguided safety test, causing meltdown and massive explosions. The radiation released killed 30 people directly and spread over northern Europe.